Although many studies demonstrate that the nucleus raphe magnus (RM) is an important relay in descending pathways that modulate nociceptive transmission, little is known about the cellular anatomy and physiology of RM neurons that serve this important function. Experiments are therefore proposed that will determine the morphology and immunocytochemistry of physiologically characterized RM cells. First, to identify how RM neurons are activated during nociception, the intracellular responses of RM neurons will be determined during reflex jaw opening evoked by tooth-pulp shock or by noxious lip heating. Neuron activation and the pattern of underlying postsynaptic potentials will be related to the sensory stimulus and to the timing and intensity of the motor response. Second, since stimulation of the periaqueductal gray region (PAG) is known to suppress nociceptive reflexes in part by activating RM neurons, the activity of RM neurons will be recorded during inhibition of reflex jaw opening produced by electrical stimulation of the PAG. RM neuron activation by tooth pulp shock and thermal stimulation will be correlated with the change in amplitude of the EMG response and the time course of the inhibitory effects of PAG stimulation on jaw opening. Third, the morphology of physiologically characterized RM neurons will be determined by labeling with intracellular HRP. The morphology and axonal projections of individual RM neurons are very heterogeneous. Intracellular labeling will allow correlation of anatomy and physiology of single cells, making it possible to determine whether the morphological features of RM neurons correspond to functional differences in discrete neuronal populations within RM. Fourth, the neurochemical content of physiologically characterized RM neurons will be determined. A number of different neurotransmitter candidates, including serotonin, substance P and somatostatin have been localized to cell bodies in RM. In order to determine the transmitter content of individual, physiologically characterized RM neurons, intracellular labeling will be combined with immunocytochemical staining. This method will determine whether RM neurons that differ electrophysiologically also differ with regard to their transmitter content. Fifth, the neurotransmitter content of afferent fibers that contact RM neurons will be determined. RM neurons receive input from afferent fibers that contact RM neurons will be determined. RM neurons receive input from afferents containing serotonin, neurotensin and somatostatin. Immunocytochemistry will be combined with intracellular labeling of RM neurons to determine which neurotransmitters are contained in afferents that contact individual, physiologically characterized RM neurons.